The present application relates to low-power portable fuel cells.
Background: Fuel Cells
A fuel cell is an electrochemical power source which is very attractive for many applications. A fuel cell may be regarded as a type of battery, but is significantly different from most common battery chemistries.
All batteries derive energy from a chemical reaction of some sort. In a fuel cell, the chemical reaction is the oxidation of a gaseous or liquid fuel (typically hydrogen), which may be supplied from an external supply. Thus, fuel cells can avoid the lifetime constraints of primary (non-rechargeable) batteries while also avoiding the degradation due to recharging and discharging which affects most rechargeable battery chemistries. The chemical reactions used in fuel cells are relatively energetic, and thus the amount of energy per unit weight is relatively high.
Much of the work on fuel cells has been directed towards larger fuel cells, in the range of a kilowatt to tens of kilowatts or more. However, the high energy density of fuel cell chemistries also makes them attractive for many portable applications, in which the energy requirements are far smaller. In particular, the development of gel-stabilized fuel cell technologies has made fuel cells much more attractive for portable applications. In such applications, the requirements of user convenience and comfort are crucial.
The oxidation of hydrogen produces water. Methanol and other hydrocarbon fuels have been proposed for fuel cells, but oxidation of any hydrocarbon fuel will produce water (as well as carbon dioxide, which is gaseous and not a problem). A fuel cell will also produce some heat, and some of the water produced will be water vapor rather than liquid water. However, some of the water vapor will condense as liquid water (either in the fuel cell plumbing, or shortly afterwards as the exhaust vapor cools). Thus liquid water will be generated.
The generation of liquid water is a significant problem: users do not want a computer which drips on their paperwork. The total flow of water is very small—on the order of one drop per minute, for 50W of power—but this is enough to be a serious nuisance in some applications.
FIG. 1 shows a typical small fuel cell for portable applications. This cell is supplied with air and hydrogen. A container 100 holds a proton transport membrane 102. The transport membrane 102 can be, for example, a sulfonated styrene/ethylene/butylene-styrene triblock copolymer from DAIS. The membrane 102 is flanked by a porous cathode 104 and a porous anode 106. (These are made of a porous conductive material, e.g. carbon fibers.) Hydrogen, supplied to fuel manifold 110 through inlet 114, is catalytically ionized at the interface between anode 106 and membrane 102. Hydrogen can then be transported through membrane 102 as protons (hydrogen ions). Similarly, oxygen is introduced through inlet 116 into oxidant manifold 112, and is absorbed at the interface between membrane 102 and cathode 104, to form oxygen ions within membrane 102. The oxygen ions and protons react to form water, which is exuded into the oxidant manifold. Typically an excess of air is pumped into inlet 116, so the exhaust port 118 carries air which is only partly deoxygenated, as well as moisture from the reaction. The free energy from the reaction can be extracted electrically at terminals V+ and V−. The voltage per cell will be in the neighborhood of 0.6V to 1.1V, depending on load characteristics and cell design.
The drawing of FIG. 1 is highly simplified. Since the membrane 102 generates only a small current per square inch, the membrane is typically folded back and forth many times. Thus the manifolds 110 and 112 will typically be long meandering passages, where condensed water can easily block gas flow. Additional pressure is therefore applied to the inputs occasionally, to produce a puff at the exhaust port which vents excess water.
Additional background on fuel cell technology can be found in Kordesh and Simader, FUEL CELLS AND THEIR APPLICATIONS (1996); the HANDBOOK OF BATTERIES AND FUEL CELLS (ed. Linden 1984); in the proceedings of the Grove Fuel Cell Symposia; and in the proceedings of the Annual Battery Conference on Applications and Advances; all of which are hereby incorporated by reference.
Innovative Portable Fuel Cell System
The present invention provides a portable fuel cell-powered system in which the water by-product is disposed of by ultrasonic vaporization. Users will object to the presence of liquid water (or to the presence of steam), but ultrasonic vaporization provides a very convenient way to expel H2O without the difficulties of handling liquid water in an office environment. Preferably a piezoelectric element is used to vaporize the water by-product, and a small port is used to eject the vapor thus produced.
In one class of embodiments, a heated airstream is combined with the water vapor exhaust port to reduce the chances of liquid water accumulating.
In another class of embodiments, the water byproduct is transported as a very-low-volume liquid flow to a vaporization orifice on the exterior of the system, where an ultrasonic transducer atomizes and expels the water.